The invention concerns a commutator of the arch-thrust type, as well as a process for the production of such a commutator.
The armoring rings of commutators of the arch-thrust type have the task of maintaining the segment set, not only in an inoperative state, but also under the thermal and dynamic load occurring during operation, under so high an arch-thrust that the individual segments cannot shift relative to each other. Shrunk rings are frequently used with such commutators as armoring rings. Customarily, these shrunk rings are placed into annular slots which must be machined into the two front ends of the segment set after the latter has been assembled from individual segments and insulating plates, provided between every adjacent segments.
However, in order to be able to perform this operation, the assembled segment set must be first pressed into a thick-walled pressure sleeve so as to have the possibility of cutting the annular slot on the turning machine into the two front ends of the segment set which is now under arch-thrust. Subsequently, the heated shrunk rings can be placed into and shrunk on the grooves at the front ends and the armored segment set, and then the set is ejected from the pressure sleeve. This production method is relatively complex and expensive, particularly since a correspondingly high tool requirement is given in the form of a great number of thick-walled pressure sleeves for a rational production of a higher number of sets and commutators.
Furthermore, the annular slots necessary to receive the shrunk rings can only be cut when the insulating plates extend into the area of the cut and, therefore, the insulating plates have, as a rule, the same shape as the copper segments. An additional disadvantage of the known commutators with shrunk rings placed in annular slots lies in the fact that the shrunk rings can only be provided at the ends of the segment set. Therefore, a high dynamic load of the commutator or a greater axial length requires not only larger dimensions for the shrunk rings but also an increase in the radial dimensions of the segments and, therefore, in the outer diameter of the commutator in order to prevent by a higher bending resistance of the segments that they can be deformed radially towards the outside or can move relative to each other in the area between the shrunk rings under the effective centrifugal force owing to too low an arch-thrust. Matters get even more complicated owing to the fact that, because the radial dimensions of the segments must be chosen of a larger size, the centrifugal force affecting them is again increased. Since such an increase in the radial height of the segments is not necessary for electrical reasons, expensive copper is used for purely mechanical reasons. Therefore, such commutators are relatively expensive so that their area of application is limited to qualitatively demanding and highly loaded electrical machines.
Two other known designs of commutators of the arch-thrust type are also rather complex even though there is no requirement of cutting annular slots into the segment set. In one design, the shrunk rings rest against the outer cover surface of the commutator with an insulation material between them and the commutator. For this reason, the commutator must be given a considerably larger axial length than would otherwise be required for electrical purposes. Additionally, the space requirement of the commutator is considerably increased by means of such shrunk rings located at the outside. In the other design, a commutator with compression molding material permits the use of armoring rings located inside, i.e. armoring rings which are located in annular L-slots extending from the inner cover surface of the segment set into the latter. Such annular L-slots in the inside are formed by punching out the segments as well as the insulating plates. The punches in the insulating plates are shaped in such a way that they keep the inserted armoring ring at a distance from the boundary surfaces of the annular L-slot formed by the segments, as is also the case for commutators with compression molding material which have armoring rings only in front-end grooves. The molding material must actually be able to penetrate into the annular L-slots in order to form there a jacket for the armoring ring. Thus, the armoring rings can only be placed into the annular L-slots without initial tension in the case of these known commutators with molding material. Tension is created by first the segment set being pressed into the mold and thus kept under arch thrust. It then expands after being filled with molding material when being ejected from the mold, necessarily its arch-thrust. The arch-thrust must be kept relatively low so that the expansion of the segment union caused by the arch-thrust does not lead to a destruction of the molded material after the ejection from the mold. Because of these reasons, the field of applications of these commutators is highly limited. Due to their relatively low arch-thrust, they are not suitable for a higher dynamically load.
A commutator is disclosed in the German Disclosure Publication 30 48 470 with a hub under initial radial tension where the segment set is provided with armoring rings being inserted under no tension as in the case of the above mentioned commutator and filled with compression molding material. This permits, therefore, only a relatively low initial tension of the hub, whereby a smooth behavior of the running surface for the brushes can also only be guaranteed under a rather low thermal and dynamic load.